Fast evaluation of theoretical uncertainties with Sherpa and MCgrid

TL;DR

This paper introduces two innovative methods, Sherpa's event-reweighting and MCgrid's interpolation grids, to rapidly evaluate theoretical uncertainties in QCD calculations, significantly speeding up PDF and $oldsymbol{ ext{alpha}_S}$ fits.

Contribution

It presents novel techniques for fast evaluation of differential cross sections in NLO QCD calculations, enhancing efficiency in theoretical error estimation and parameter fitting.

Findings

Event-reweighting in Sherpa enables quick recalculations for varied parameters.

MCgrid's interpolation grids support fastNLO tables for rapid cross section evaluations.

Methods significantly reduce computational time for PDF and $oldsymbol{ ext{alpha}_S}$ fits.

Abstract

The determination of theoretical error estimates and PDF/$\alpha_S$-fits require fast evaluations of differential cross sections for varied QCD input parameters. These include PDFs, the strong coupling constant $\alpha_S$ and the renormalisation and factorisation scales. Beyond leading order QCD, a full dedicated calculation for each set of parameters is often too time-consuming, certainly when performing PDF-fits. We report on two methods to overcome this issue for any QCD NLO calculation: The novel event-reweighting feature in Sherpa and the automated generation of interpolation grids using the recently introduced MCgrid interface. For MCgrid we present the newly added support for fastNLO tables and highlight some future developments.